ACAS II is an aircraft system based on Secondary Surveillance Radar (SSR) transponder signals. ACAS II interrogates the Mode C and Mode S transponders of nearby aircraft (‘intruders’) and from the replies tracks their altitude and range and issues alerts to the pilots, as appropriate. Non-transponding aircraft are not detected.

ACAS II works independently of the aircraft navigation, flight management systems, and Air Traffic Control (ATC) ground systems. While assessing threats it does not take into account the ATC clearance, pilot’s intentions or autopilot inputs.

If a risk of collision is established by ACAS II, an RA will be generated. Broadly speaking, RAs tell the pilot the range of vertical speed at which the aircraft should be flown to avoid the intruder. The visual indication of these rates is shown on the flight instruments. It is accompanied by an audible message indicating the intention of the RA. A "Clear of Conflict" message will be generated when the aircraft diverge horizontally.

The vertical sense (direction) of the RA is coordinated with other ACAS II equipped aircraft via a mode S link, so that two aircraft choose complementary manoeuvres. RAs aim for collision avoidance by establishing a safe vertical separation (300 – 700 feet), rather than restoring a prescribed ATC separation.

ACAS II operates on relatively short time scales. The maximum generation time for a TA is 48 seconds before the Closest Point of Approach (CPA). For an RA the time is 35 seconds. The time scales are shorter at lower altitudes (where aircraft typically fly slower). Unexpected or rapid aircraft manoeuvre may cause an RA to be generated with much less lead time. It is possible that an RA will not be preceded by a TA if a threat is imminent. The effectiveness of an RA is evaluated by the ACAS equipment every second and, if necessary, the RA may be strengthened, weakened, reversed, or terminated.

A protected volume of airspace surrounds each ACAS II equipped aircraft. The size of the protected volume depends on the altitude, speed, and heading of the aircraft involved in the encounter. See illustration below.

RAs can be generated before ATC separation minima are violated and even when ATC separation minima will not be violated. In Europe, for about two thirds of all RAs, the ATC separation minima are not significantly violated.

Types of RAs (TCAS II version 7.0)

Complying with RAs

Pilots are required to immediately comply with all RAs, even if the RAs are contrary to ATC clearances or instructions.

If a pilot receives an RA, he/she is obliged to follow it, unless doing so would endanger the aircraft. Complying with the RA, however, will in many instances cause an aircraft to deviate from its ATC clearance. In this case, the controller is no longer responsible for separation of the aircraft involved in the RA.

On the other hand, ATC can potentially interfere with the pilot’s response to RAs. If a conflicting ATC instruction coincides with an RA, the pilot may assume that ATC is fully aware of the situation and is providing the better resolution. But in reality ATC is not aware of the RA until the RA is reported by the pilot. Once the RA is reported by the pilot, ATC is required not to attempt to modify the flight path of the aircraft involved in the encounter. Hence, the pilot is expected to “follow the RA” but in practice this does not yet always happen.

International Standard

The International Civil Aviation Organization (ICAO) is responsible for the global standardisation of ACAS.

ACAS equipment is available from three vendors. While each vendor’s implementation is slightly different, they provide the same core functions and the collision avoidance and coordination logic contained in each implementation is the same. In order to be certified, ACAS equipment must meet the Minimum Operational Performance Standards (MOPS) laid down set in RTCA and forthcoming EUROCAE documents.

The equipment which meets the ACAS II Standards and Recommended Practices (SARPs) is known as TCAS II, version 7. A joint RTCA/EUROCAE working group is currently finalising amendments to the MOPS, addressing three specific safety improvement changes related to the collision avoidance logic; these new MOPS will form TCAS II version 7.1.

Types of ACAS

ACAS I

Gives Traffic Advisories (TAs) but does not recommend any manoeuvres. The only implementation of ACAS I concept is TCAS I. ICAO Standards and Recommended Practices (SARPs) for ACAS I are published in ICAO Annex 10, volume IV and are limited to interoperability and interference issues with ACAS II.
ACAS I is mandated in the United States for certain smaller aircraft

ACAS II

Gives Traffic Advisories (TAs) and Resolution Advisories (RAs) in the vertical sense (direction). The only implementation of ACAS II concept is TCAS II Version 7. ICAO SARPs published in Annex 10. (Link to ICAO Provisions)
ACAS II (TCAS II at software version 7.0) is mandated in Europe. (Link to Equipage Requirements)

Equipment Requirement

In Europe, as of 1 January 2005 all civil fixed-wing turbine-engined aircraft with a maximum take-off mass over 5,700 kg, or capable of carrying more than 19 passengers, must be equipped with TCAS II version 7.0. Additionally, many State and business aviation aircraft are also equipped.

Safety Benefits

The safety benefits delivered by ACAS are usually expressed in terms of the risk ratio (does ACAS make safety better or worse?). For Europe, ACAS is estimated to reduce the risk of mid-air collision by a factor of about 5 (i.e. a risk ratio of 22%).

The importance of correctly following RAs is illustrated by the fact that a pilot who never follows RAs faces three times the risk that is faced by a pilot who always follows RAs. The human is the weakest element in the ACAS control loop; without “human in the loop” the risk ratio would improve by a factor of 10.